1. Field of the Invention
The present invention relates to a laser unit to be used in a laser scanning apparatus for performing an optical writing by using a laser beam, such as a laser beam printer (LBP) and a laser facsimile (FAX) machine.
2. Related Background Art
Conventionally, a laser scanning apparatus for performing an optical writing scan to a photosensitive body or the like, such as a LBP, is constructed as shown in FIG. 1. In FIG. 1, a collimated light beam emitted from a laser unit 30, which is comprised of a laser light source, a collimator lens, a laser driver circuit and the like, is reflectively deflected by a rotating polygonal mirror 31, and a reflectively deflected light beam is scanned on a surface of a photosensitive drum (not shown) through a scanning lens 32 and a return mirror 33.
The collimated light beam is scanned within the width of the drum as a beam optimally condensed by the scanning lens 32. A portion of the scanned beam is reflected by a BD mirror 34 and detected by a BD sensor 35 in order to perform a synchronous detection of the optical writing and prevent a positional shift in the optical writing. A cylindrical lens 37 is generally used to prevent a positional shift in up and down directions (i.e., a sub-scan direction) of the beam on the photosensitive body due to an inclination error of the polygonal mirror surface. Thus, the beam from the laser is condensed with respect to the sub-scan direction on the polygonal mirror surface, and a focused linear image is formed thereon. The polygonal mirror surface and the photosensitive body surface are set conjugate with each other with respect to the sub-scan direction by the scanning lens 32. Further, all those members are mounted onto an optical box 36 within tolerances in size by using reference pins and the like. The laser unit 30 is shown in more detail in FIG. 2.
A semiconductor laser 40 is mounted to a metal base member 45 together with a laser driver circuit 48. At the time of assembly, the base member 45 and a holder 44 are appropriately positioned with respect to a direction of an alignment plane while the laser is driven by the laser driver circuit 48, so that the optical axis of a collimator lens 42 mounted to the side of the holder 44 and a radiation point of a laser tip 41 disposed in a package of the semiconductor laser 40 are aligned with each other, and those members are fixed by screws 47 after the alignment is completed. The alignment contains adjustments in Y--Y' and Z--Z' (in-and-out of the page) directions in FIG. 2. The collimator lens 42 is moved along its optical axis (the X--X' direction) to convert the laser light beam to a collimated beam, and thus the collimator lens 42 is positioned in conjugate relation with the radiation point of the laser tip 41. Therefore, the collimator lens 42 is held in a lens barrel 43 by a push ring 46. Focusing is conducted by moving the barrel 43 along the optical axis (the X--X' direction) in the holder 44, and when alignment is complete, an adhesive is injected through a drip port 49 to fix the lens barrel 43 to the holder 44. Thus, the laser unit 30, which is assembled as a unit, is mounted at a reference position of the optical box 36.
In the prior art apparatus, however, the size of the entire apparatus is difficult to reduce because the laser unit used in the laser scanning apparatus is built by simply assembling a plurality of parts, such as a metal base member and a holder. Furthermore, the laser driver circuit is made up of a print-circuit board on which electronic components are disposed and hence is large in shape. At the same time, the wiring distance for the semiconductor laser is relatively long. As a result, the apparatus is not suited for a high-frequency optical writing in terms of frequency characteristics.